The present invention relates to a motor controller.
Conventionally, a type of a motor controller has been used which converts direct-current voltage supplied from a direct-current power supply to three-phase (U, V, W) drive power using a PWM (pulse-width modulation) inverter, and supplies the three-phase drive power to a brushless motor.
As shown in FIG. 3, a PWM inverter is configured of three arms 52 in parallel as basic units. Each arm 52 corresponds to one of the phases and includes a pair of switching elements (for, example, power MOSFETs) 51A, 51B in series. The motor controller alternately turns on and off the high voltage side switching element 51A and the low voltage side switching element 51B of each arm 52 at a predetermined timing, thereby supplying three-phase drive power to a brushless motor 53.
In such a motor controller, to prevent a short circuit (arm short circuit) between each high voltage side switching element 51A and the corresponding low voltage side switching element 51B, “dead time” is set when the switching elements 51A, 51B are turned on and off, in which both switching elements 51A, 51B are off. However, the dead time causes errors to occur between a voltage command value and an output voltage of the PWM inverter. This in turn creates torque ripples, vibrations, and current distortion, which can cause noise. Therefore, a number of types of motor controllers executes “compensation” for reducing errors between a voltage command value and an output voltage due to dead time, thereby preventing current distortion. See Hidehiko Sugimoto et al. Facts of Theory and Design of AC Servo Motor Systems, 6th edition, Denshi Shuppansha, August 2002, pp. 56–58.
For example, as shown in FIG. 4, a method has been proposed in which a dead time compensation amount β is added to or subtracted from a DUTY instruction value αx. The DUTY instruction value αx is compared with a triangular wave δ, which is a carrier wave, for determining the timing of turning on and off the switching elements 51A, 51B. The dead time compensation amount β is set in advance according to the direction of current of the DUTY instruction value αx.
More specifically, when the direction of current of X-phase (X=U, V, W, the same applies hereinafter) that corresponds to one of the arms 52 is toward the brushless motor 53 from the arm 52, that is, when the direction of current is “positive” (see FIG. 3), the dead time compensation amount β is added to the DUTY instruction value αx. When the direction of current is toward the arm 52 from the brushless motor 53, that is, when the direction of current is “negative” (see FIG. 3), the dead time compensation amount β is subtracted from the DUTY instruction value αx. This equalizes a time in which the X-phase output voltage Vx becomes the power supply voltage Vb (t3+t4 or t5+t6) in a cycle T of the triangular wave 5 with the corresponding time (t1+t2) in a case where no dead time is set (ideal voltage waveform). Thus, the voltage command value is caused to match the output voltage of the PWM inverter, so that current distortion due to dead time is prevented.
Other than the method described above, a method has been proposed in which the output voltage of PWM inverters are detected, and feedback control is executed so that the values of the output voltage match a voltage command value. This method also prevents current distortion due to dead time.
However, the switching elements forming a PWM inverter vary in response speed. Further, the switching elements vary in temperature characteristics and aging characteristics, which depend on the usage environment. Therefore, the above introduced method in which the DUTY instruction value αx is corrected using the dead time compensation amount β, compensation amounts of the phases and directions of current can be insufficient or excessive depending on the usage environment. This may lead to a situation where current distortion due to dead time is not prevented with accuracy. On the other hand, the method using feedback control, a delay is inevitable in providing compensation.